The present invention relates generally to a method and apparatus for determining blood pressure and more particularly to a non-invasive method and apparatus for determining blood pressure, and for providing the medical practitioner with information about the operation of the cardiovascular system of a patient.
One test often performed on patients by medical practitioners is a test to determine the blood pressure of the patient. Blood pressure is tested often because a knowledge of a patient's blood pressure provides an overall reflection of the functioning of his heart and circulatory system.
The blood pressure in a patient's arterial system is represented by the peak systolic and diastolic levels of the pressure pulse and is modified by cardiac output, peripheral arteriolar resistance, distensibility of the arteries, amount of blood in the system, and viscosity of the blood. Accordingly, changes in blood pressure reflect changes in these measurements. For example, the decrease in vessel distensibility in the elderly lowers diastolic pressure and increases systolic pressure to produce systolic hypertension. Increases in blood volume may raise both systolic and diastolic components.
Normal blood pressure in the aorta and large arteries, such as the brachial artery, varies between 100 and 140 millimeters of mercury mm Hg systolic and between 60 and 90 mm Hg diastolic. Pressure in the smaller arteries is somewhat less, and in the arterioles, where the blood enters the capillaries, it is about 35 mm Hg. However, a wide variation of normal blood pressure exists, and a value may fall outside the normal range in healthy adults. The normal range also varies with age, sex and race. For example, a pressure reading of 100/60 may be normal for one person but hypotensive for another.
Arterial blood pressure can be measured directly or indirectly. The most common known method for measuring blood pressure indirectly is with a sphygmomanometer and stethoscope. The primary benefits of the sphygmomanometer and stethoscope procedure are that it is simple for the medical practitioner to use, is non-invasive and is relatively inexpensive. The sphygmomanometer and stethoscope are often sufficiently inexpensive to make their cost well within the reach of consumers who desire to perform blood pressure tests at home. The primary drawback of the use of the sphygmomanometer and stethoscope procedure resides in the limited amount of data that it provides, and the relative inaccuracy of the procedure.
The procedure by which a sphygmomanometer is utilized to determine blood pressure is relatively simple. A collapsed, inflatable blood pressure cuff is affixed snugly and smoothly to a patient's arm, with the distal margin of the cuff at least 3 cm above the anticubital fossa.
Pressure in the cuff is then rapidly increased to a level of about 30 mm Hg above the point at which the palpable pulse disappears. As the cuff is deflated, observations may be made either by palpation or auscultation. The point at which the pulse can be felt is recorded from the manometer as the palpatry systolic pressure.
The ausculatory method is usually preferred to the technique described above. With this method, vibrations from the artery under pressure, called Korotkoff sounds, are used as indicators.
To determine blood pressure using the ausculatory method, the bell or diaphram of a stethoscope is pressed lightly over a brachial artery while the cuff is slowly deflated. The pressure readings begin at the time the Korotkoff sounds first become audible. As the cuff is deflated further, the sounds become louder for a brief period. The sounds then become muffled and finally disappear. The systolic blood pressure is the point at which the Korotkoff sounds become audible, and the diastolic blood pressure is the point at which the sounds cease to be heard. The traditional manual sphygmomanometer may provide inaccurate blood pressure measurements because it relies too much on human hearing sensitivity and the experience of the operator.
In addition to the "manual" method described above for indirectly measuring blood pressure, several electronic devices exist which measure blood pressure according to the same theory as discussed above. One of these devices is the MARSHALL ASTROPULSE 78 Model blood pressure measuring device which is manufactured by the Marshall Medical Corporation of Lincolnshire, Illinois.
One problem with the electronic methods discussed above is that they do not provide very accurate measurements of blood pressure.
Electronic devices may not be able to measure all patients' blood pressure accurately because electronic devices usually depend upon some pre-assumed signal conditions for determining blood pressure. For example, female patients typically have a thicker fat layer than male patients. This thicker fat layer can lead to a less accurate blood pressure measurement in female patients.
Another difficulty encountered with the above-described indirect blood pressure measurement techniques is that they only provide a rather limited amount of information. Specifically, they do not provide significant information relating to the dynamics of the cardiovascular system of the patient, such as information relating to the volume of blood flowing through the cardiovascular system and the operation of the valves of the heart.
Another method for measuring blood pressure is by a direct measurement technique. In a direct measurement of arterial blood pressure, a needle or catheter is inserted into the brachial, radial, or femoral artery of the patient. A plastic tube filled with heparinized saline solution connects the catheter to a pressure sensitive device or a strain-gauge transducer. The mechanical energy that the blood exerts on the transducer's recording membrane is converted into changes in electrical voltage or current that can be calibrated in millimeters of mercury. The electrical signal can then be transmitted to an electronic recorder and an oscilloscope, which continually records and displays the pressure waves.
This direct measurement technique is more accurate than the indirect sphygmomanometer method, and yields an electrically integrated mean pressure. However, as will be appreciated, the direct measurement technique has several drawbacks. The invasive nature of the technique makes it more difficult for the practitioner, as well as less convenient, and more traumatic for the patient.
One other method for indirectly measuring blood pressure is disclosed in Geddes, et al., "The Indirect Measurement of Mean Blood Pressure in the Horse," THE SOUTHWESTERN VETERINARIAN, Summer, 1970, p. 289-294. The Geddes article describes the indirect measurement of blood pressure through the oscillometric method.
The oscillometric method is concerned with the amplitude of the oscillations communicated to a cuff encircling a body member containing a suitably large artery. During deflation of the cuff from above systolic pressure, a sequence of oscillations on the cuff-pressure indicator can be seen. At suprasystolic pressure, small oscillations in cuff pressure are evident. When cuff pressure falls just below systolic pressure, the amplitude of the oscillations increases. With continued deflation of the cuff, the oscillations grow in amplitude, reach a maximum, and then decrease continually until the cuff pressure drops below the diastolic pressure of the patient. Currently, several digital-display electronic blood pressure monitors are commerically available which utilize the oscillometric method. One such commerically available device is the NORELCO.RTM. brand HC-3001 Model home use blood pressure measuring device which is manufactured by the North American Phillips Corporation of Stamford, Connecticut.
The Geddes article also discusses the measurement of mean arterial pressure (MAP). MAP is defined as the average pressure that pushes blood through the circulatory system in the human body. True MAP is not the arithmetic average of systolic and diastolic pressure, but rather depends on the height and contour of the arterial pressure wave. True MAP is dependent upon a patient's Cardiac Output (CO) and the Total Peripheral Resistance (TPR) of the patient's cardiovascular system. Due to this relation, the equation EQU MAP=(CO) (TPR)
is often used by medical practitioners to describe MAP.
MAP is believed by many practitioners to be the most important measurement of blood flow through the circulatory system. It is essential for a practitioner to know the patient's MAP when deciding whether to prescribe medicines for the patient to control hypertension. For a further discussion of MAP determinations, see the Geddes article.
The device described in Geddes consists of a battery operated electronic oscillometer which displays cuff pressure and the oscillations in cuff pressure. Within the device is a pressure transducer, an amplifier and a rapidly responding meter which displays only the amplitude of the oscillations in cuff pressure. A gain control is provided to adjust the amplitude of the display of the oscillations. Additionally, an auxilliary output jack is provided to permit the oscillations to be recorded on a graphic recorder. Geddes utilized the oscillometer discussed above simultaneously with a direct pressure measuring device on the same animal at the same time to compare the accuracy of his indirect measurements with the direct measurements. Geddes found that his indirect oscillation method was not as accurate as the direct method, with the average ratio of indirect to direct pressure being about 0.92:1.
Another indirect method and apparatus for measuring blood pressure is described in Ramsey U.S. Pat. Nos. 4,360,029 and 4,349,034. The '029 and '034 patents are related, and disclose generally identical subject matter.
The Ramsey patents relate to an automatic indirect blood pressure reading method and apparatus which automatically and adaptively pump up an arm cuff. The cuff is pumped to a proper pressure by taking the previous cuff pressure measurement and adding approximately 60 mm to the old pressure before beginning measurement of the amplitude of the oscillations in the cuff. Once the amplitude of oscillations at the starting pressure are measured, the cuff is deflated a determined pressure increment to a lower pressure. The oscillations at this lower cuff pressure are then measured.
Ramsey requires that the pressure oscillations satisfy a plurality of artifact detecting tests before a peak oscillation measurement is accepted as valid. Should an artifact be detected, additional oscillations are measured until the oscillations tested free of artifacts. When this integrity test is satisfied or some predetermined time interval is exceeded, the cuff is once again deflated a pressure increment. The apparatus continues in this fashion until maximum amplitude oscillations are obtained at the lowest cuff pressure, which is indicative of the mean arterial pressure.
Notwithstanding the above discussed advances in the indirect measurement of blood pressure, room for improvement exists. It is therefore one object of the present invention to provide a method and apparatus for measuring blood pressure which provides the medical practitioner with a more accurate method of measuring blood pressure, and which provides the practitioner with information regarding the cardiovascular system beyond that of mere systolic and diastolic arterial pressures.